The present invention relates to a process for pretreating a hydrocarbon conversion catalyst based on an EUO zeolite. The present invention also relates to the activated catalyst obtained and to its use in particular in the field of processes for isomerising aromatic compounds containing 8 carbon atoms.
The catalyst activated using the process of the invention can be used for isomerising aromatic compounds containing 8 carbon atoms. In known processes for isomerising aromatic compounds containing eight carbon atoms, a feed which is generally low in para-xylene with respect to the thermodynamic equilibrium of the mixture (i.e., in which the amount of para-xylene is substantially lower than that of the mixture at thermodynamic equilibrium at the temperature under consideration, this mixture comprising at least one compound selected from the group formed by meta-xylene, ortho-xylene, para-xylene and ethylbenzene), and generally rich in ethylbenzene with respect to this same mixture at thermodynamic equilibrium, is introduced into a reactor containing at least one catalyst, under suitable pressure and temperature conditions to obtain from the reactor outlet a composition of C8 aromatic compounds which is as close as possible to the composition of said mixture at thermodynamic equilibrium at the temperature of the reactor.
The para-xylene and optionally meta-xylene or ortho-xylene, which are the desired isomers as they are of great importance in particular for the synthetic fibre industry, are then separated from this mixture.
However, there are a number of problems associated with isomerising C8 aromatic compounds, caused by secondary reactions. Thus in addition to the principal isomerisation reaction, hydrogenation reactions are observed, such as hydrogenation of aromatic compounds to naphthenes, and naphthene ring opening reactions which lead to the formation of paraffins containing at most the same number of carbon atoms per molecule as the naphthenes from which they are formed. Cracking reactions are also observed, such as paraffin cracking which leads to the formation of light paraffins, typically containing three to five carbon atoms per molecule, also dismutation and transalkylation reactions which lead to the production of benzene, toluene, aromatic compounds containing nine carbon atoms and heavier aromatic compounds. All together, those secondary reactions are highly deleterious to the yields of the desired products and depend on the catalyst used for the C8 aromatic compound isomerisation reaction.
The skilled person is well aware that in certain catalytic processes, activation and/or selectivisation procedures have to be carried out on the catalyst to optimise the catalyst performance.
As an example, in the case of a catalyst containing a metal from group VIII of the periodic table, pretreating the catalyst with hydrogen sulphide (H2S) is well known. The sulphur contained in the hydrogen sulphide molecule fixes itself to the metal and endows it with improved catalytic properties.
The Applicant""s French patent application FR 98.04651 describes a process for activating a C8 aromatic compound isomerisation catalyst by sulphurisation and passivation with ammonia. International patent application WO-A-92/13046 describes an activation process comprising passivating a hydrocarbon catalyst based on a Y zeolite with ammonia. U.S. Pat. No. 5,321,184 describes a process for activating a C8 aromatic compound isomerisation catalyst based on pentasil zeolite by pretreatment under particular conditions.
The present invention provides a process for pretreating a catalyst used in hydrocarbon conversion processes, comprising the following steps:
a) pretreating a catalyst containing at least one zeolite with structure type EUO and at least one hydrodehydrogenating metal in the presence of a hydrocarbon feed, at a temperature such that a catalyst comprising carbon is obtained;
b) then treating the hydrocarbon feed and the catalyst at a temperature which is lower than the temperature applied in step a).
The present invention also concerns the activated catalyst obtained and the use of the activated catalyst in a process for isomerising a feed comprising xylenes and ethylbenzene.
The pretreatment of the invention can activate the catalyst by coating the active sites with a layer of carbon. In accordance with the invention, the catalyst comprises at least one zeolite with structure type EUO and at least one hydrodehydrogenating metal from the periodic table, and the layer of carbon deposited can attenuate the activity of the metallic and acidic sites, so that when using said catalyst in isomerising aromatic C8 compounds, the loss of aromatic C8 compounds by secondary side reactions of dismutation, transalkylation, hydrogenation and cracking can be reduced, while the selectivity is improved and good ethylbenzene conversion is retained. Further, the use of the process of the invention also comprising sulphurisation and optionally also comprising passivation with ammonia enables the catalyst to be used at lower temperatures and at higher HSVs (weight of feed/weight of catalyst/hour).
The present invention concerns a process for pretreating a catalyst for converting hydrocarbon feeds comprising at least one EUO zeolite and at least one hydrodehydrogenating metal from the periodic table by applying severe temperature conditions when starting off the hydrocarbon conversion process, for a duration sufficient to deposit a certain quantity of carbon on the catalyst, preferably at least 0.1% by weight of carbon with respect to the total catalyst weight, and more preferably at least 0.3%. Following this treatment, the catalyst continues to be brought into contact with the hydrocarbon feed to be converted but under less severe temperature conditions than those applied during the pretreatment.
Further, in a particular implementation of the invention, the pretreatment step can comprise using a particular pressure. Thus, preferably, the pressure applied in step a) is higher than the pressure applied in step b) for the same hydrogen/hydrocarbon ratio.
In this particular implementation, step a) is then characterized in that at least one parameter selected from temperature and pressure is at a value such that the P, T set adopted leads to more favourable conditions for the formation of carbon compared with the conditions used in step b) during which the hydrocarbon conversion reaction proper is carried out.
Pretreatment step a) is carried out for a period of 5 to 120 hours, preferably 20 to 96 hours, the second step b) of the reaction proper being initiated after this first step.
Thus, for example, step a) can be applied at a temperature which is at least 20xc2x0 C. higher than the temperature of step b), preferably at least 30xc2x0 C. higher.
Further, the pressure can be varied. Thus, for example, step a) can be carried out at a pressure which is equal to or higher than the pressure of step b), preferably at least 0.3 MPa higher than the pressure in step b), under identical H2/HC ratio conditions.
In one implementation of the invention, the catalyst is sulphurised. The catalyst is sulphurised using a sulphur compound, for example hydrogen sulphide or a hydrogen sulphide precursor. The catalyst is sulphurised before the pretreatment of the invention, without the feed and can, for example, be carried out before introducing said catalyst into the reactor, the catalyst then being termed a xe2x80x9cpre-sulphurised catalystxe2x80x9d. It can also be carried out using a catalyst which is already in place in the reactor.
In general, before sulphurisation, the compound of the hydrodehydrogenating metal contained in the catalyst is reduced. This pre-sulphurisation step can be carried out with pure hydrogen sulphide or with a precursor, preferably an organic precursor, of hydrogen sulphide which is then decomposed in the reactor.
Non limiting examples of sulphur-containing organic compounds which can be used in the sulphurisation step of the present invention are alkyl sulphide compounds, aryl sulphide compounds and alkylaryl sulphide compounds. Examples are butylethylsulphide, diallylsulphide, dibutylsulphide, dipropylsulphide, dimethyldisulphide (DMDS), thiophene, dimethylthiophene and ethylthiophene.
The catalyst sulphurisation step is normally carried out in a neutral or reducing atmosphere at a temperature of about 20xc2x0 C. to 500xc2x0 C., preferably about 200xc2x0 C. to 400xc2x0 C., at an absolute pressure of about 0.1 to 5 MPa, preferably about 0.3 to 3 MPa, and with a volume of gas (inert or reducing) per volume of catalyst per hour (HSV) of about 50 hxe2x88x921 to 600 h1xe2x88x92, preferably about 100 hxe2x88x921 to 200 hxe2x88x921.
The inert gas is usually nitrogen and the reducing gas is usually hydrogen, most often substantially pure hydrogen.
In a further implementation of the process of the invention, the catalyst is passivated with ammonia. Passivation can be carried out before or after the sulphurisation step and these steps preferably take place before the pretreatment. Preferably, the sulphurisation step is carried out before the passivation step. These two, sulphurisation and passivation, steps can be carried out before or after introducing the catalyst into the reactor. Preferably, the passivation step in the presence of ammonia is carried out when the catalyst is already in place in the reactor and thus simultaneously with the pretreatment under severe temperature conditions and possibly severe pressure conditions.
Ammonia passivation is usually carried out in two stages: injecting a quantity of at least a certain amount of ammonia, in the form of gaseous NH3, or in the form of at least one ammonia precursor, then continuous injection of ammonia in the form of gaseous NH3 or in the form of at least one ammonia precursor on introducing the feed to be converted, and preferably at the same time as catalyst activation. Preferably, the first injection is carried out with gaseous NH3 and the second injection is carried out with at least one ammonia precursor.
The ammonia precursors which can be used in the present invention are all compounds which are known to the skilled person which, in the presence of hydrogen, decompose to ammonia which is fixed on the catalyst. Compounds which can be used which can be cited include aliphatic amines such as n-butylamine.
Further, the catalyst can be reduced and reduction is carried out in the presence of hydrogen, preferably more than 90 mole % pure. The reduction temperature is about 300xc2x0 C. to 550xc2x0 C., preferably about 400xc2x0 C. to 520xc2x0 C. The total pressure is in the range from atmospheric pressure to 3 MPa, preferably about 0.5 to 2 MPa. The duration of the reduction step is normally about 1 to 40 hours, pressure about 1 to 8 hours. The hydrogen flow rate (addition of fresh hydrogen and hydrogen recycled from the reactor outlet to the reactor inlet) is about 0.1 l/h/g to 100 l/h/g of catalyst.
The activated catalyst of the process of the present invention comprises at least one EUO zeolite, i.e., EU-1, TPZ-3 and ZSM-50 zeolites.
EU-1 zeolite with structure type EUO, which has already been described in the prior art, has a unidimensional microporous network with a pore diameter of 4.1xc3x975.7 xc3x85 (1 xc3x85=1 Angstrom=1xc3x9710xe2x88x9210 m) (xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, W. M. Meier and D. H. Olson, 4th edition, 1996). Further, N. A. Briscoe et al. have disclosed in an article in the review Zeolites (1988, 8, 74) that these unidimensional channels have lateral pockets with a depth of 8.1 xc3x85 and a diameter of 6.8xc3x975.8 xc3x85. The mode of synthesis of EU-1 zeolite and its physico-chemical characteristics have been described in European patent EP-B1-0 042 226. U.S. Pat. No. 4,640,829 concerns ZSM-50 zeolite, which has structure type EUO according to the xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, W. M. Meier and D. H. Olsen, 4th edition, 1996. EP-A1-0 051 318 deals with TPZ-3 zeolite which has structure type EUO according to xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, W. M. Meier and D. H. Olson, 4th edition, 1996.
The zeolite with structure type EUO is at least partially in its acid form, and comprises silicon and at least one element T selected from the group formed by aluminium, iron, gallium and boron, preferably aluminium, with a global Si/T atomic ratio of more than 5.
The catalyst also comprises at least one matrix which comprises at least one compound selected from the group formed by clays, magnesia, aluminas, silicas, titanium oxide, boron oxide, zirconia, aluminium phosphates, titanium phosphates, zirconium phospates and silica-aluminas. Preferably, the matrix is alumina.
The catalyst comprises at least one hydrodehydrogenating element from the periodic table, preferably selected from group VIII metals and group VIB metals selected from palladium, platinum, nickel, iron, cobalt, chromium, manganese, tungsten, vanadium and molybdenum and can also optionally contain at least one metal selected from metals from groups IIIA and IVA, preferably selected from tin an indium.
The catalyst comprises, by weight with respect to the total catalyst mass:
1% to 90% of at least one zeolite with structure type EUO, preferably 3% to 60%, more preferably 4% to 40%;
0.01% to 2% by weight of at least one hydrodehydrogenating metal, preferably 0.05% to 1%;
optionally, 0.01% to 2% of at least one additional metal selected from groups IIIA and IVA of the periodic table, preferably 0.05% to 1%;
optionally, sulphur;
a binder making up the complement to 100% of the catalyst.
In a preferred implementation of the invention, a catalyst comprising a zeolite with structure type EUO is used wherein the crystal size is less than 5 micrometres (xcexcm), normally less than 0.5 xcexcm and usually less than 0.2 xcexcm. These crystals or crystallites are normally arranged as aggregates with a grain size such that the value of Dv,90 is less than or equal to 500 xcexcm, normally less than 400 xcexcm, usually less than 200 xcexcm and preferably less than 50 xcexcm. The aggregate size is determined by laser diffraction granulometry. This measurement is made on the zeolite powder suspended in water. After an initial measurement, the suspension is subjected to ultrasound for thirty seconds then a new measurement is made. The ultrasound used is characterized by a power of 50 W and a frequency of 50 kHz. This procedure is repeated until the result no longer changes (xc2x15%). The size distribution of the aggregates defined by volume is calculated from the light signals collected by the detectors and using the Fraunhofer theory. Dv,X is defined as the diameter of the equivalent sphere such that the size of X% by volume of aggregates is less than said diameter. These characteristics are obtained directly during synthesis of the zeolite and/or by any method enabling the aggregate size to be reduced, such as post-synthesis grinding or suitable mixing before forming.
Further, the hydrodehydrogenating metal dispersion of the activated catalyst of the present invention is preferably in the range 50% to 100%, more preferably in the range 60% to 100% and still more preferably in the range 70% to 100%; the macroscopic distribution coefficient of said hydrodehydrogenating metal is in the range 0.7 to 1.3, preferably in the range 0.8 to 1.2. Preferably, the catalyst is formed into beads or extrudates and its strength is such that its bed crushing strength is more than 0.7 MPa.
The invention also concerns the use of the catalyst in a process for isomerising a feed comprising at least one xylene isomer selected from para-xylene, ortho-xylene and meta-xylene, and ethylbenzene.
In this implementation of the invention, the catalyst pretreatment process is carried out in the same reaction zone as the isomerisation reaction for the xylenes and ethylbenzene.
Thus in the case where the activated catalyst is used in a process for isomerising aromatic compounds containing 8 carbon atoms, the activated catalyst is characterized in that it comprises carbon and in that pretreatment step a) is carried out under severe temperature and pressure conditions. Thus, for example, the pretreatment of step a) is carried out at a temperature in the range 400xc2x0 C. to 500xc2x0 C., radical at a temperature in the range 410xc2x0 C. to 450xc2x0 C. Further, the pressure can be varied. As an example, a total pressure in the range 1 to 2 MPa can be applied during step a), preferably in the range 1.2 to 1.8 MPa.
The isomerisation process is then carried out during step b) using any method which is known to the skilled person. As an example, the isomerisation reaction temperature during step b) is about 300xc2x0 C. to 410xc2x0 C., preferably about 320xc2x0 C. to 400xc2x0 C. and more preferably about 350xc2x0 C. to 400xc2x0 C.; the partial pressure of hydrogen is about 0.3 to 1.5 MPa, preferably about 0.4 to 1.2 MPa; the total pressure is about 0.45 to 1.9 MPa, preferably about 0.6 to 1.5 MPa; the HSV (weight of feed/weight of catalyst/hour) is about 0.25 to 15 hxe2x88x921, preferably about 1 to 10 hxe2x88x921, and more preferably about 2 to 7 hxe2x88x921.
In this implementation of the invention, the catalyst contains at least one metal from group VIII of the periodic table, preferably platinum or palladium.
In a preferred implementation of the isomerisation process of the present invention, along with the feed to be isomerised and the hydrogen required for the reaction, at least one compound with a boiling point in the range about 80xc2x0 C. to 135xc2x0 C. is introduced, more particularly at least one compound selected from the group formed by paraffins containing eight carbon atoms per molecule, benzene, toluene and naphthenes containing eight carbon atoms.
This or these compounds are added to the feed to be isomerised as a recycle, in the form of fresh compounds or in the form of a mixture of recycled and fresh compounds in quantities such that the percentages by weight of added compounds with respect to the total feed entering the reactor are normally as follows:
the percentage by weight of paraffins containing eight carbon atoms, in the optional case when they are added to the feed, is about 0.1% to 10%, preferably about 0.2% to 2%;
the percentage by weight of naphthenes containing eight carbon atoms, in the optional case when they are added to the feed, is about 0.5% to 15%, preferably about 2% to 8%;
the percentage by weight of toluene, in the optional case when it is added to the feed, is about 0.1% to 10%, preferably about 0.2% to 5%;
the percentage by weight of benzene, in the optional case when it is added to the feed, is about 0.1% to 10%, preferably about 0.2% to 2%.
The percentage by weight of the total mass of added compounds, when adding a plurality of compounds, is normally about 0.1% to 20% by weight, preferably about 2% to 15% by weight with respect to the total feed entering the isomerisation zone.
The following examples illustrate the invention without in any way limiting its scope.
The following abbreviations are used in the examples in the present description: xe2x80x9cC1-C8 paraffinsxe2x80x9d for paraffins containing 1 to 8 carbon atoms; xe2x80x9cC5-C9 naphthenesxe2x80x9d for naphthenes containing 5 to 9 carbon atoms; and xe2x80x9cC9-C10 aromatic compoundsxe2x80x9d for aromatic compounds containing 9 and 10 carbon atoms.